An anti-lock braking system (ABS) on a motor vehicle prevents the wheels from locking while braking. ABS allows a driver to maintain steering control during periods of heavy braking by preventing a skid and allowing the wheel to continue to forward roll. A typical ABS has a controller, a speed sensor for each wheel, and a braking circuit. The controller controls the braking applied to the wheels in order to make them either turn faster or slower. This process is repeated constantly during braking. The brake torque is repeatedly increased and decreased in a cyclical fashion.
Longitudinal forces on the vehicle are controlled with ABS. However, it is known that the maximum amount of longitudinal force on a vehicle is typically proportional to the amount of normal force on the tires during heavy braking. When the brakes are applied at a rapid rate, the normal forces on the vehicle change substantially and quickly. In an under-damped vehicle, the normal forces can continue to oscillate throughout the stop. Much of ABS philosophy strategy assumes that normal forces and the pressures to generate wheel lock are nearly constant during a typical stop. The variation in normal forces, while well known, is typically not quantified nor is it typically used to modify brake control.
Empirical methods are typically used to manage the first cycle of ABS control. Some applications have implemented logic to limit the rate of brake applied in order to reduce the perturbance to vehicle motion. The control behavior is tuned to minimize deviations from optimum control that are observed during development testing. The current method of tuning is often used to compensate for several factors that affect control in addition to normal force variation such as; consistency of the calculation of a reference velocity, hysteresis in the brake torque/pressure relationship, the tire/road μ, the optimum deceleration of the vehicle, whether the vehicle is on split-μ, and the motion of the tire relative to the general motion of the vehicle. The addition of a sound analytical basis to account for normal force variation provides a more optimum tuning because separate parameters are used to account for normal force variation.
Several current brake control algorithms limit the rate of brake torque development during the initial application of brakes. This method reduces the rate of change of normal forces. With lower normal force variation, the control is generally subjected to less variation and achieves higher efficiency. However, in slowing the rate of brake applied, the development of deceleration is slowed and stopping distances are increased.
There is a need for a model that determines how much normal force will change on a vehicle in response to the brake applied and take into account how normal force is varying in order to modify ABS control in anticipation of the normal force in order to support optimum modulation of braking torque.